DE3239718A1 - QUICK SINTER STEEL AND METHOD FOR THE PRODUCTION THEREOF - Google Patents

Description

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description

The invention relates to a new sintered high-speed steel with high vanadium content, which has an excellent combination of high hardness and ductility, and a powder metallurgical process for its manufacture.

High-speed steel is distinguished from other ¹ tool steels by its great hardness when hot. It is the preferred tool material for a variety of cutting and shaping operations. It is used for drill bits, boring cutters, end mills, drills, milling cutters, cutting tools, reamers, punching tools, shear blades and the like. Another widely used tool material are tungsten carbide co-sintered hard metals. High-speed steels are superior to cemented carbide in terms of their ductility, even when fully hardened, but show an inferior hardness. There is therefore a need for high speed steels with properties between those of conventional high speed steels and those of cemented carbides.

High speed steel comprises a matrix of martensite in which carbides of the type M, C, M ", C", MC (where M stands for an or several metals or alloys) are finely dispersed, the ductility of which is mainly due to the properties the matrix and its hardness is determined by the carbide content. He contains. W, Mo, Cr, V, Co, C and Fe as the remainder as the main components with a nominal composition of 10 to 24% (weight), W + 2Mo (W equivalent), 4% Cr, 1 to 5% V, 0 to 17% Co, less than 2% Mn + Si, remainder C and Fe (where C generally with Is calculated using the following equation:

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C = 0.19 + 0.017 W equivalents + 0.2 - 0.22 V (%)), wherein W and Mo mainly carbides of the type M, C, Cr mainly carbides of the type M., .. C, and V mainly Carbides of the type MC (it being assumed that it is present as VC or as V.C-. In steel) and the Total carbide content is in the range of 20 to 30%.

There have repeatedly been unsuccessful attempts to increase the carbide content. On the one hand, the increase in the content of carbides of the type M, C is carried out by Increasing the W equivalent (and also the carbon content) over the above range from a rapid drop in ductility due to the deterioration of the Accompanied by the microstructure. On the other hand, an increase in the content of carbides of the MC type is prohibited by increasing the vanadium content (and also the carbon content j due to difficulties in melting, since at the same time an increase in the melting temperature and a The solidus-liquidus area has been broadened. Furthermore, billets tend to have an increased carbide content, in particular with a vanadium content of more than 5% in hot forging to break, in that the coarse carbide nets break open, formed along grain boundaries during solidification.

According to a recently proposed and economically used spray method, a jet is a molten Alloy at such a high speed that the formation of coarse carbide is suppressed Drip is cooled, which is then either in a capsule by hot forging or by hot isostatic pressing compacted into solid bars. This process has the advantage that one can rely on the forging step mentioned above can do without, but is subject to restrictions that result from the spraying of the melt mil.

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hem vanadium guhalL and a crushing deformation of the Ingots result, so that it is not possible to increase the vanadium content to more than 6.5%.

The present invention is based on the finding that
the incorporation of vanadium carbide, which, after being introduced into the matrix, acts as an ideal hardener that only
is little affected by the presence of other carbides and the composition of the matrix, is prevented in the conventional borrowed methods that they all start from an alloy melt. The following will
therefore a process described which is based only on solid reactions and which makes it possible to introduce as much Vana medium as desired, so that in this way

a high-speed steel with a high vanadium content is formed, which has an increased hardness and a little decreased te ductility.

The invention therefore relates to a hard yet ductile sintered high-speed steel with a high vanadium content, the properties of which lie between those of a conventional cutting tool steel and a cemented carbide and which has the following composition:

1.4 to 6.2% C,

10.0 to 24.0% W + 2Mo,

3.0 to 6.0% Cr,

8.5 to 38% V,
less than 17% Co,

Remainder Fe and unavoidable impurities.
30th

The invention also relates to a method
for making the hard yet ductile high speed sintered steel of the composition

1.4 to 6.2% C,
10.0 to 24.0% W + 2Mo,

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3.0 to 6.0% Cr,

8.5 to 38% V,

less than 17% Co,

Remainder Fe and unavoidable impurities, which is characterized by the fact that the alloy components in the form of powdery oxides with powdery Carbon or graphite (hereinafter referred to as carbon for the sake of simplicity) mixed, the mixture heated in a hydrogen stream and in this Convert the mixture through the added carbon and the flowing hydrogen into an alloy powder at the same time reduced, the alloy powder pulverized and thereby making the necessary adjustments or adjustments to the composition, 'the alloy powder' into one pressed compacted body, sintered the compacted body in a vacuum, the sintered body, if necessary, hot isostatic presses and finally converts the matrix of the sintered body into martensite by a heat treatment.

Another object of the invention is a simple method for producing the hard yet ductile Composition sintered high speed steel

1.4 to 6.2% C,

10.0 to 24.0% W + 2Mo,
3.0 to 6.0% Cr,

8.5 to 38 % V, less than 17% Co,

Balance Fe and unavoidable impurities, which control the grain size of the vanadium carbide in the steel made possible and characterized in that the alloy components are in the form of powdery Oxides mixed with powdered carbon and adjusts the vanadium oxide content to a low value, the mixture is heated in a stream of hydrogen and

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thereby the mixture through the added carbon and the flowing hydrogen is reduced at the same time to an alloy powder, the reduced alloy powder with powdery Vanadium carbide enriched to the desired value, pulverized the mixture obtained, and if necessary makes corrections to the carbon content, presses the mixture into a compacted body, sintering the compacted body in a vacuum, optionally hot isostatic pressing of the sintered body obtained and finally converting the matrix of the sintered body into martensite by heat treatment.

The sintered high-speed steel according to the invention is characterized is characterized in that it contains exceptionally large amounts of finely divided carbides of the type MC in uniform distribution in the matrix and an increased hardness and has a little reduced ductility.

The permissible ranges for the various alloy components are common for conventional high-speed steels. They are therefore also used according to the invention, apart from the fact that the inventive In terms of its composition, high-speed steel differs from conventional high-speed steel increased content of vanadium and associated carbon. The increase in the vanadium content does not affect the approved ranges for the other alloy components. This is because vanadium represents the strongest carbide former in steel and its carbide is like an independent component in the matrix behaves, which is little influenced by the presence of other elements. Albeit vanadium in any In amounts that can be added, it is desirable to keep its content below 38%. Machining up to an addition of up to 20%

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teres and still possible up to an addition of 25%. The Grinding becomes difficult with added amounts of 38%, while above this the tendency to become brittle and to one There is loss of ductility. Regarding the lower It should be noted that there are significant advantages to adding vanadium occur at an amount of about 8.5%, as illustrated by Example 3 below.

The cutting tool steel according to the invention is with the help a powder metallurgical process, in which the production of a sinterable alloy powder is essential. The alloy powder is produced by first converting the alloy components into Form of the powdery oxides with powdery carbon mixed, then the mixture to a particle size of less than 10 μm, preferably less than 5 μm, ground and finally reduced in a hydrogen stream. In this context it should be noted that the reduction of the oxide mixture with the aid of carbon or water hydrogen used alone at such a high temperature that a liquid phase can occur, so that a Grain growth and a clustering of the reduced Particles can occur to such an extent that the subsequent pulverization is impaired.

The invention is based here on the knowledge that with the simultaneous presence of carbon and hydrogen the reduction can be carried out at such a low temperature that grain growth occurs is practically avoided. The invention is based on the further knowledge that simultaneously with the reduction alloying of the constituents can be achieved.

Carbon becomes an excess of the oxide mixture added for the purpose of solution and carbide formation, where-

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where the excess amount corresponds to half of the theoretical amount necessary to reduce the oxides to carbon monoxide, with the hydrogen contributing the remaining half. It goes without saying that these amounts can be modified as a function of the specific reduction conditions, taking into account the hydrogen supply rate, the heating rate and heating time, the dimensions of the furnace used, etc. For the reduction, heating to about 1000 ^° C. for three hours is generally sufficient Alloy powder should preferably contain less than 1% residual oxygen. The removal of the remaining oxygen and / or the increase in the proportion of the dissolved carbon can, if desired, be achieved in the subsequent sintering process by an additional addition of carbon on the basis of the composition analysis of the reduced powder. Although the removal of excess dissolved carbon can in principle be achieved in a similar manner by adding additional oxides to the reduced powder, this generally leads to control difficulties and a deterioration in the quality of the sintered product. Thus, the reduction conditions should rather be set so that the reduced powder is slightly shifted from the fully reduced state to the carbon deficit side.

The reduced powder is again reduced to a particle size of less than 10 μm, preferably less than 5 μm, pulverized, making the necessary adjustments to the carbon content, after which it is with a suitable binder, for example paraffin, is added, compacted and sintered. The removal of the Waxing can be effected independently or at an early stage of the sintering. The heating takes place in a vacuum or in a non-oxidizing atmosphere of less

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than 0.133 mbar (0.1 mmHg) in order to promote the removal of the gases (predominantly carbon monoxide) from the compressed product / in particular at temperatures between 900 and HOO ⁰ C. When selecting the sintering temperature in the solid phase range, longer heating times at low temperatures and accelerated grain growth at high temperatures must be taken into account. Depending on the composition of the alloy, the sintering temperature is generally in the range of 1050 ⁰ C (high vanadium contents) to 125O ⁰ C (low vanadium contents), at sintering times ranging from 1 hour to 2 hours. The densities of the product formed should generally exceed 95% of theoretical density. You can start the sintering process at a density of about. Finish 95% and then hot-isostatically press the material in order to achieve full density, especially if high ductilities are desired, or you can increase the sintering temperatures in order to achieve densities of 98 to 99% if the high hardness compared to the good Ductility is preferred.

The heat treatment is carried out in conventional manner, that is, the austenitizing at about 1200 ⁰ C, the cooling in the air, which is optionally interrupted by austempering at about SOO ⁰ C to liberate large sintered body of thermal stresses, and two to three times tempering at Temperatures between 500 and 600 ^° C. for converting the remaining austenite into martensite and for promoting carbide precipitation in the matrix.

Another method of making a high vanadium alloy powder is through the oxide mixture with a low vanadium content and then the reduced product with powdered vanadium carbide to enrich. The main advantage of this two-stage vanadium carbide enrichment is in addition to the

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Simplification of the reduction of the oxide mixture in that the grain size of the MC-type carbide can be controlled in relation to that of the matrix; H. fine MC granules in fine matrix granules or relatively coarse MC granules in

.5 fine matrix granules, a measure not possible with the method described above. There are however, situations where coarse carbide grains are preferred over fine carbide grains and vice versa. For example, the former show at high sliding speeds when dry, a greater wear resistance than the latter.

The invention is described in more detail below with reference to FIG the accompanying drawings explained. In the drawings show:

Fig. 1 is a graph of the transverse breaking strength and

Fig. 2 of hardness

of alloys rich in vanadium, their

Vanadium content in the basic composition SKH57 was varied according to the invention, and

Figure 3 is a photomicrograph of a hot isostatic pressed alloy having a Va

nadium content of 20% in the quenched condition.

The following examples serve to further illustrate the invention.

example 1

For the production of an alloy powder with a composition that corresponds to the JIS standard SKH 57 (10% W,

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3.5% Mo, 4% Cr, 3.5% V, 10% Co, j 1.2 5 "% C, remainder Fe), but has an increased content of V and C (20% and 4.88, respectively %) are mixed 1.261 kg WO ,, 0.525 kg MoO ₃ , 0.585 kg Cr ₂ O ₃ , 2.942 kg V ₃ O ₃ , 1.271 kg CoO and 6.808 kg ^Fe 2 ° 3 (which Fe ₃ O ₃ 0.4% Si and contains just as much Mn), each in the form of a powder with a particle size of 5 to 10 μm, intimately with 2.428 kg of carbon black, which has been pulverized in a ball mill to a particle size of less than 5 μm, after which the material is removed without a binder formed into pellets, heated quickly in a hydrogen stream to 1050 ^° C. and held at this temperature for 3 hours. The reduction conditions used are:

Load: 10 kg
Dimensions of the furnace (box type): 128 1 hydrogen feed rate: 0.23 L / min heating rate: 4 ⁰ C / min.

Of the 2.428 kg of added carbon, 1.94 kg form half of the theoretical 3.88 kg required to reduce the metal oxides to CO, while the remaining 0.488 kg are dissolved. The alloy powder obtained has a bulk density of 1.0 g / cm ³ and contains 1.2% residual oxygen and 3.80% dissolved carbon. The pelletized alloy powder can easily be pulverized to the original particle size, carbon being corrected by adding 1.98% carbon, 0.9% of which is intended for removing the residual oxygen and 1.08% for further dissolution.

The adjusted alloy powder mixed with 4% paraffin is used to form test pieces with a thickness of 6 mm, a width of 10 mm and a length of 30 mm by compaction, after which they are at 0.07 mbar (0.0 ') Torr)

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sinters. Effected prior to sintering for 90 minutes at 1180 ⁰ C to a dewaxing at 300 ⁰ C and a degassing at 900 to 1100 ⁰ C. In this manner, the for 40 minutes at 1150 obtained a sintered body having a density of 96%, ⁰ C and 981 bar (1000 atmospheres) is brought to a density of 100% by hot isostatic pressing, after which an austenitizing heat treatment is carried out for 3 minutes at 1110 ⁰ C, cooled in the air and heated three times for 2 hours at 560 ⁰ C 10. pert. .

To determine the extent to which the mechanical properties are influenced by the vanadium content and the hot isostatic pressing, samples are prepared in a manner similar to that described above, which contain 3 to 40% vanadium and are then tested for their transverse breaking strength (FIG. 1) and Hardness. (Fig. 2) examined. The symbols "a" and "a ¹ " shown in FIG. 1 stand for samples which have been formed with and without hot isostatic pressing, a differentiation which disappears in FIG. Although the increase in hardness is accompanied by a slow decrease in ductility, a high transverse breaking strength of 210 to 230 kg / cm ^2, as in conventional cutting steels, is maintained with an alloy according to the invention with a vanadium content of 35% without hot isostatic pressing. The beneficial effect of hot isostatic pressing on ductility is evident, especially in the low vanadium range. The transverse rupture strengths of samples obtained without hot isostatic pressing but through increased sintering temperatures lie between curves "a" and "a ¹ " in FIG. 1, which indicates the possibility of being able to do without hot isostatic pressing in those cases , since a high transverse breaking strength is not necessary

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is. By adding 10% V or more, hardnesses can be achieved that exceed that of the CIS V4 Co cemented carbide, which is 66 HRC. It has been found that alloys containing 10 to 15% V, a tendency to fracture own, if they are not hot forged between 900 and 1100 ⁰ C. This means that further densification of these alloys with a high vanadium content is only possible by using hot isostatic pressing. 3 shows a photomicrograph (magnification 400) of a hot isostatically pressed alloy according to the invention with a vanadium content of 20% in the quenched state, which shows the uniform dispersion of fine VC carbide particles.

B e i s ρ i e 1 2

A different procedure is used to prepare the alloy of Example 1 containing 20% V. One uses the same amounts of the metal oxides as given in Example 1, with the exception of V "O-, and mixed they intimately with 1.6 kg of carbon black, after which the material is pulverized to a density of less than 5 μπ \ and under the Reduced the same conditions as indicated in Example 1. The analyzes show a residual oxygen content of 1.1% and a dissolved carbon content of 0.2% in the reduced powder. The powder is then with 0.06 kg carbon and 2.470 kg powdered vanadium carbide (Particle size 7 μπι) added, further mixed and pulverized to a particle size of below 5 .μπι.

The subsequent measures, such as compacting, the Sintering, hot isostatic pressing and heat treatment are carried out in the manner indicated in Example 1. There are no differences in hardness, transverse breaking strength and microstructure between the samples, which have been prepared from the powder of Example 1

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and determine the sample of this example.

When using a reduced alloy powder according to the invention wants to apply, its content of dissolved carbon must preferably be as low as possible, since the total amount from this carbon and that introduced by the added vanadium carbide, the desired value can exceed what, however, from the carbon and vanadium contents depends on the agent added and the material to which the additive is added. When an excess of carbon It is feared it is recommended to use a non-stoichiometric VC with low carbon content too or use the excess carbon in the subsequent sintering stage due to the residual oxygen content of the alloy powder to consume.

Example 3

The alloys of Example 1 are prepared with vanadium contents of 3.5, 7.5 and 8.5% turning tools or turning teeth with a square cross-section of 10 mm and compares them with regard to their turning behavior under Use of a SUS 27 rod with a diameter of 50 mm, whereby at a speed of 390 min, one

25- feed speed of 0.25mm / revolution and one Cutting depth of 2.5 mm works. A cutting fluid is used. The turning tool is shaped so that the tip clamping angle 10 °, the side approach angle 15 °, the rear angle 6 °, the backward cutting angle 5 °, the The side cutting edge angle is 5 ° and the edge radius is 2 mm. Compare cutting tool life on the basis of the machined axial distance as a function of the flank wear. The turning tool off the alloys with a vanadium content of 3.5 resp. 7.5% allows only 12 mm, while the turning tool

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With a vanadium content of 8.5%, the permissible wear limit when examined at 38 mm is not yet has reached. Other comparisons of wear resistance will show changes in the undercut angle (0. °), the side cutting edge angle (10 °) and the edge radius (1 mm). In this case the alloys with a vanadium content of 3.5% or 7.5% at 35 mm fail, while the turning steel from the Alloy with a vanadium content of 8.5% enables a good surface finish even at 75 mm. . ■

There are no changes in the results depending on the results whether the turning tools are hot isostatically pressed or not. From this it can be seen that the desired The effect of the vanadium enrichment is clearly manifested with an addition of 8% or more.

Although a commercially available sprayed high-speed steel (ASP 60 TM) contains only 6.5% vanadium, it turns out to be the turning steel with a vanadium content of 7.5% as superior and comparable to the turning tool according to the invention with a vanadium content of 8.5%, contrary to the expectation that the cutting behavior or. The higher the vanadium content, the greater the rotational behavior is. However, the compared alloys differ both in terms of their composition as well as in terms of their manufacturing method. The fact that the wear resistance increases with increasing vanadium content has been confirmed by additional investigations Findings of turning steels of the same basic composition with a vanadium content of 10 and 15% are confirmed.

Example 4
From the alloys of Example 1 with a vanadium

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hold of 3.5% and 15%, two-headed end mills with a diameter of 10 mm are formed and compared with regard to the end face milling of an SKD 11 tool steel block made of HRC 23 at a speed of 580 min, a feed speed of 51 mm / min and a cutting depth of 9 mm, working without cutting fluid. The service life is compared depending on the milling distance until the point in time when the tools show a flank wear of 0.08 mm. The end mill from of the alloy with a vanadium content of 3.5% this wear at 800 mm, while the end mill made of the alloy with a vanadium content of 15% at 1600 mm has only a flank wear of 0.03 mm and thus more than 500% better performance than the End mill made from the alloy with a vanadium content of 3.5%.

As described in Examples 1 to 4, the dispersion-reinforced alloy structure can be used Cutting tool steels not only in terms of their composition speak alone, without referring to the content and morphology of the dispersed materials (according to the invention carbides of type MC), d. H. on the manufacturing method through which the properties and performances an alloy can be influenced to a great extent. For example the alloy of Example 1 with a vanadium content of 3.5% has a similar composition like the alloy SKH57, but shows a significantly higher transverse breaking strength than the latter, the has been manufactured by a melting process.

Albeit woIfram carbide tools on a large scale and with Success that can be applied to most metal cutting operations is high speed steel generally for the machining of cast iron

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iron, aluminum, titanium and alloys thereof are more suitable, especially for interrupted machining operations. Ductilities of TRS 210 to 230 kg / mm ² are more than sufficient for machining, although the use of high-cut steel is impaired by its low hardness. So far, however, it has not been possible to improve the hardness or the wear resistance by increasing the carbide content without reducing the ductility. According to the invention, methods are now specified with which it is possible to increase the vanadium content up to 38% and thus a combination of large To achieve hardness and only slightly reduced ductility in high-speed steels.

The powder metallurgical method according to the invention has there are still considerable advantages over the conventional production of high-speed steels. For example disposable inserts and the like have been manufactured by machining base materials. However, the increase in the carbide content leads to Difficulties in manufacture and increases the cost of machining and the devices required for it and offsets the benefit of the tool's better performance. By powder metallurgical methods these manufacturing problems on those of powder compaction reduced, whereby the previous restrictions can be overcome.

Claims (5)

PATENTANWÄLTE TER MEER-MÜLLER-STEINMEISTER Representatives admitted to the European Patent Office - Professional Representatives before the European Patent Office Mandatalres agrees pres! 'Office european des brevets Dipl.-Chem. Dr. N. ter Meer Dipl.-Ing. H. Steinmeister Dipl.-lng, FE Müller Artur-Ladebeck-Strasse Tnftstrasse 4, D-8OOO MÜNCHEN 22 D-48OO BIELEFELD tM / cb December 7, 1982 Case 1611 German patent application P 32 39 718.6 (PCT / JP82 / 00113) The Furukawa Electric Co., Ltd. No. 6-1, 2-chome, Marunouchi, Chiyoda-ku, Tokyo, Japan Kanto Denka Kogyo Co., Ltd. No. 2-1, 1-chome, Marunouchi, Chiyoda-ku, Tokyo, Japan Fujidie Co., Ltd. No. 17-10, 2-chome, Shimomaruku, Oota-ku, Tokyo, Japan High vanadium sintered high speed steel and method of making it Priority: April 08, 1981, Japan, No. 56-52709 International filing date: April 08, 1982 Claims 1. Sintered high speed steel with high vanadium composition
1.4 to 6.2% C, 10.0 to 24.0% W + 2Mo (W equivalent), 3.0 to 6.0% Cr, 8.5 to 38% V, TER MEER · MÜLLER · sVeFnMEISTeH ···· * .. *.! .. Case 1611 less than 17% Co, Remainder Fe and unavoidable impurities. 2. Process for the production of a sintering high-speed steel with high vanadium composition 1.4 to 6.2% C, 10.0 to 24.0% W + 2Mo, 3.0 to 6.0. % Cr,. 8.5 to 38% V, less than 17% Co, Remainder Fe and unavoidable impurities, characterized in that the Alloy components in the form of powdery oxides with powdered carbon or graphite (hereinafter for the sake of simplicity called carbon) mixed, the mixture heated in a hydrogen stream and thereby the mixture by the added carbon and the flowing hydrogen at the same time to an alloy powder reduced, pulverizing the alloy powder with the necessary composition settings, pressing the alloy powder into a compacted body, sintering the compacted body in a vacuum, and finally converting the matrix of the sintered body into martensite through a heat treatment. 3. A method of making a sintered high speed steel with a high vanadium content of the composition 1.4 to 6.2% C, 10.0 to 24.0% W + 2Mo,
3.0 to 6.0% Cr, 8.5 to 38% V,
less than 17% Co, Remainder Fe and unavoidable impurities, characterized in that mandie Alloy components in the form of powdery oxides TERMEER-MOLLER-STBlNMElSTBfJ! ..: .. '.. * .'... Case 1611 ~ " Yes ~ with powdered carbon or graphite (hereinafter called carbon for the sake of simplicity), the mixture is heated in a stream of hydrogen and thereby the Mixture by the added carbon and the flow reduced the hydrogen to an alloy powder at the same time, the alloy powder pulverized with the necessary setting of the composition, the alloy powder presses into a compacted body, sintering the compacted body in a vacuum, the obtained sintered body hot isostatic presses and finally the matrix of the sintered body by a heat treatment in martensite converts. 4. Process for the production of a sintered high speed steel with high vanadium content of the composition 1.4 to 6.2% C, 10.0 to 24.0% W + 2Mo,
3.0 to 6.0% Cr, 8.5 to 38% V, less than 17% Co, Remainder Fe and unavoidable impurities, characterized in that one the alloy components in the form of the powdery Oxides mixed with powdered carbon and the vanadium oxide content is set to a low value, the mixture is heated in a stream of hydrogen and thereby the mixture by the added carbon and the flowing hydrogen at the same time to one Alloy powder reduced, the reduced alloy powder with powdered vanadium carbide on the Enriching the desired value, pulverizing the mixture obtained with the necessary carbon correction that The mixture is pressed into a compacted body, the compacted body is sintered in a vacuum and finally the Matrix of the sintered body by heat treatment TER MEER · MÖLLER ■ STe! NMEISTSE * J.! .. Case 1611 converts to martensite. 5. A method of making a high speed sintered steel of the composition
1.4 to 6.2% C, 10.0 to 24.0% W + 2Mo, 3.0 to 6.0% Cr,
8.5 to 38% V,
less than 17% Co,
Remainder Fe and unavoidable impurities, characterized in that the alloy components in the form of the powdery oxides are mixed with powdery carbon and the vanadium oxide content is set to a low value, the mixture is heated in a hydrogen stream and thereby the mixture is caused by the added carbon and the flowing hydrogen at the same time reduced to an alloy powder, the reduced alloy powder enriched with powdered vanadium carbide to the desired value, pulverized the mixture obtained with the necessary carbon correction, pressed the mixture into a compacted body, sintered the compacted body in a vacuum, hot isostatic pressing the obtained sintered body and finally converting the matrix of the sintered body into martensite through a heat treatment.